In 1978, 50 Hawaiian monk seals died mysteriously. Biologists thought the critically endangered mammals had been poisoned by ciguatoxins, a family of cyclic polyethers made by subtropical marine plankton and gobbled up by herbivorous fish that the seals eat. But scientists lacked the tools to directly measure the toxin. Now biologists have discovered the toxin in living seals using a noninvasive test developed to detect ciguatoxins in human blood (Environ. Sci. Technol.,DOI: 10.1021/es2002887).

The combined NOAA team found that about a fifth of the 55 free-ranging, seemingly healthy animals in the wild had high levels of ciguatoxin in their blood, according to the cytotoxicity assay and liquid chromatography/mass spectrometry analysis. The levels reached several picograms of toxin per milliliter of blood, a dose high enough to poison a rat. But three animals in shoreline pens, which ate different foods, had no toxin in their blood.

This study is the first detection of ciguatoxin in monk seals using a noninvasive method, says corresponding author John Ramsdell. The team now hopes to see if long-term effects of the toxin explain some of the decades-long decline in monk seals' numbers. The researchers also want to see whether sharks, which prey on seals, carry the toxin.